The manner in which Mr. Mateen pledged loyalty, or baya, points to him being a lone wolf, or at the very least, newly indoctrinated. In his 911 call, Mateen reportedly pledged allegiance to Islamic State itself, rather than claiming his attack in the name of a certain cleric or leader such as self-anointed “caliph” Abu Bakr al-Baghdadi, as is custom for jihadis.

Nonetheless, the Amaq News Agency, a semi-official media arm of IS, later announced that the group claimed responsibility for the actions of an "Islamic State fighter," and on Monday, the IS radio station, al-Bayan, named Mateen as “one of the soldiers of the caliphate in America.”

The Islamic State's embrace of Mateen reflects its confidence in what counterterrorism experts say has become the group’s preferred mode of attack in the US – a no-risk, high-reward tactic that instantly gives the group media exposure and expands its reach into the heart of America.

The increasing reliance on “lone wolves” has also become a tactical necessity, experts say, as potential IS supporters in the US face several obstacles preventing them from traveling to IS territory in Iraq and Syria to receive training or arms.

“The difference between an inspired attack and a directed attack is training – which ISIS can’t do in the US, so it is relying on the former,” says Patrick Skinner, a former CIA case officer and counterterrorism expert at The Soufan Group. “The last Ramadan statement said specifically to attack the West – you don’t have to come to Syria, to the caliphate – just go out and attack.”

A popular image

IS supporters were quick to promote the Orlando attack on social media, posting a flurry of images of wolves with the hashtag #OrlandoExplosion in Arabic, promising further IS-inspired terror.

In one split-screen showing an IS fighter set to behead a prisoner and a wolf’s head, the caption reads: “the West has lone muscles, Islam has lone wolves,” while another stated, “I am the wolf that terrorizes the land of the infidels … it is a duty to eliminate the treacherous.”

Al Krar (Twitter handle @alkrarr__), another IS supporter, had perhaps the most ominous post: a graphic of a shower head raining water over a brain stating: “We’ll enter your kids’ and family’s brain and make them Islamic State supporters”

Call to arms

The Orlando shooting comes a few short weeks after IS spokesman Abu Mohamed al-Adnani called on ISIS supporters to carry out a series of “devastating” attacks in the US during the holy month of Ramadan, which began June 6 and runs through early July.

Vowing a month of “calamities,” Mr. Adnani urged followers to strike “the heart” of Europe and the US.

“This call specifically goes out to the supporters of the Islamic Caliphate in Europe and America…. The smallest action you do in the heart of their land is dearer to us than the largest action by us,” Adnani said in a recorded statement released May 21.

In the case of the Paris attacks, where 11 men targeted six sites, IS used operatives who received training and material support from Syria, and unleashed maximum carnage in a carefully orchestrated plot.

With "inspired" lone-wolf attacks, however, IS is most likely unable to control the target or the timing of the attack, or ensure its success, counterterrorism experts say.

The attack last December on a regional center in San Bernardino, Calif., had no symbolic significance to the group, yet the deadly shooting succeeded in striking fear and uncertainty in the United States. Another IS supporter last year, however, targeted a Garland, Texas, event depicting Mohammad cartoons.

The location of the Orlando attack, meanwhile, may provide an instance of perfect convergence of Mateen's personal hatred and IS's radical beliefs. Mateen had expressed deep anger toward gays, aligning him with IS, which brutally executes suspected homosexuals in its territories and spews antigay rhetoric. Its followers celebrated the shooting on social media, calling for further attacks on LGBT individuals.

Difficult to track

The IS approach poses a difficult challenge to security specialists. Counterterrorism and law-enforcement agencies in the West have grown accustomed to the Al Qaeda model, which requires months of training and planning.That gives counterterrorism agencies or US allies in the Middle East more opportunity to discover them.

But in this case, the FBI had twice investigated Mateen, first in 2013 for “inflammatory statements” to coworkers suggesting extremist ties, and again in 2014 for potential links to Moner Mohammed Abusalha, a fellow Fort Pierce, Fla., native who became the first American to carry out a suicide attack in Syria.

With no credible links to a terrorist group, no known contacts with terrorist agents or travel to countries where extremist cells are active, the FBI lacked evidence to pursue its investigation. Had Mateen been a follower of Al Qaeda, experts say, his record would have likely showed some connections that would have raised red flags.

“Al Qaeda remains a very deliberate, at times slow, organization that has a top-down and controlled approach, which can be tracked by security agencies. ISIS is decentralized,” says Hassan Abu Haniya, an Amman, Jordan-based expert in jihadist groups.

The best way to prevent copycat lone wolves, short of curbing civil liberties at home, say counterterrorism analysts, may be degrading ISIS’s ability abroad, destroying its image as a self-proclaimed caliphate.

The Orlando attack came at a time IS is losing territory in Sirte, Libya, and Minbaj, Syria, and is fending off an offensive by Iraqi forces threatening to drive it from Fallujah. However, the group will capitalize on the Orlando attack – and any future lone wolves – as rare good news for the group.

“The narrative yesterday was that ISIS was losing in Libya, losing in Syria, and close to losing Fallujah,” Mr. Skinner said. “Today the headline is that ISIS is rising and carried out the worst terrorist attack since 9/11 – and they didn’t even know the guy who did it.”